1. Field of the Invention
The present disclosure is a formulation and synthesis of an energetic polymer. The synthesis of this energetic polymer can be altered to modify the mechanical properties, energy content, and oxygen balance of the final energetic polymer.
2. Description of Related Art
Tough, elastomeric polymers such as polybutadiene have long found use as components in composite, solid propellants. These polymers, end-capped with hydroxyl groups, can be crosslinked with diisocyanates such as isophorone diisocyanate (IPDI) to yield a binder material capable of safely accommodating reactive propellant ingredients. This binder provides the mechanical resistance necessary for the active components to withstand stimuli such as impact and heat. If tailored with plasticizers such as DOA (di-octyl adipate), this binding can greatly reduce the sensitivity of highly reactive propellant systems.
Although the mechanically robust polyunsaturated hydrocarbon binders like polybutadiene can effectively provide a matrix component with reduced sensitivity function, their low energy content and minimal combustibility decrease the overall energy density and performance otherwise available to unbound propellant mixtures. For this reason, several “energetic” binder materials have been developed to achieve higher energy densities at equivalent or lower levels of reactive fillers. These materials almost universally seek to achieve higher endothermicity by attaching pendant nitrato and/or azido groups to a polyether backbone. Common energetic binders which utilize pendant azido groups include polyAMMO (poly(3-azidomethyl-3-methyl oxetane), polyBAMO (poly(3,3-bis-azidomethyl oxetane), and GAP (glycidyl azide polymer). Common energetic binders which utilize pendant nitrate groups include polyNIMMO (poly(3-nitrato-methyl-3-methyloxetane) and polyGLYN (polyglycidyl nitrate).
Many of the solid propellants used in missile and rocket propulsion systems currently in use or development by the Army include an inert polymeric binder matrix composed of urethane crosslinked poly-unsaturated hydrocarbons. By incorporating the propellant solid ingredients (i.e. oxidizers, metal fuels, explosive fillers and ballistic modifiers) within these tough and flexible binder matrices, otherwise sensitive munitions can be made insensitive, or at least less sensitive, to mechanical stimuli such as friction, impact, and electrostatic discharge.
These polybutadiene and urethane derived binding networks lack the stored chemical energy characteristic of the high-energy compounds necessary for a munition's functionality. This lack of energy contributes to an overall decrease in the energy content and density impulse of the final energetic material system. Ideally, a polymeric binder should be developed that has a higher energetic functionality and density than polybutadiene, yet remains inert, tough, flexible, and safe during all conditions except desired ignition. During desired ignition, the binder matrix's energy content should contribute significantly to the total energy production of the propulsion system. In this way, a reduction of hazardous high energy filler loading would be possible while maintaining the same level of propellant performance. Alternatively, for any given level of filler, a greater performance would be achieved substituting an energetic binder for a non-energetic binder.
Approaches currently used to synthesize other energetic polymers employ chemistry that cannot easily tailor the transition temperature (Tg), mechanical properties, oxygen balance, or energetic content of the resulting polymers. As such, common energetic polymers must utilize additives such as plasticizers to achieve the right balance of Tg and mechanical properties, while their oxygen balance and energetic content remain fixed. For example, GAP-based propellants do not exhibit good mechanical properties, suffering especially from poor low temperature properties. This drawback has to be managed by heavy loading of plasticizers or by blending with flexible linear-structural polymers such as PEG and PCL.
This excessive plasticization is not desirable due to possible side effects such as reduced shelf life/reliability resulting from plasticizer migration or a reduced energy density stemming from the large volume of unreactive plasticizer. This reduction in energy density can be minimized by using various energetic plasticizers; however, the issue involving plasticizer migration and shelf life remains. Current research to sidestep the necessity for energetic binder plasticization involves copolymerizing various energetic polymers in a way that creates a thermoplastic elastomer binder with augmented mechanical properties. So far, this approach has met with some success; however, plasticizers still must be used to achieve the low-temperature properties necessary for a successful energetic binder.